Ive added some code to the forum which will make it easier to link to items on ebay. When you make a new post you will now see an 'ebay' button above the text box, this will produce some code that looks like this

[ ebay ] [ /ebay ]

Just put the item number in the middle like so

[ ebay ] 12345458785 [/ebay ]

and it should produce a listing that looks like this, it's embedded in the post and is 'clickable'